Apparatus for correcting single bit insertion or deletion in a data payload with a checksum corrector

ABSTRACT

This application discloses a message format including a data payload of N bits and a corrector component encoding a checksum to correct the checksum of single bit slipping noise, where the checksum is the sum of each data payload bit by its position modulo N+1. The corrector component may encode a second checksum derived from the checksum that may also be included in the message and so on. Apparatus embodiments may include a transmitter generating a transmitted message of this format and/or a receiver using a received message that may be corrupted from the transmitted message through bit slipping in the form of bit insertion or bit deletion.

TECHNICAL FIELD

This invention relates to the correction of data payloads where thetransmission error is a single bit inserted or deleted.

BACKGROUND OF THE INVENTION

There are communication systems that rely on the synchronization oftransmitter and receiver clocks to correctly transfer data payloads. Oneclass of such systems involves transmitters and receivers separated bylarge distances, such as satellite communications with a ground station.The clocks at the transmitters and receivers may drift occasionally,causing single bits to be inserted or deleted between the data payloadthe transmitter tried to send and what is received.

FIG. 1A shows an example of a prior art message 2 including a datapayload 4 of N bits and a checksum 6 of J bits as in a Cyclic-RedundantCode (CRC). Such prior art error correction schemes assume the receivedchecksum to be uncorrupted in almost all bit positions.

If the bits of the message 2 are treated alike, then insertion ordeletion of a bit is equally probable at any bit location and thereceived data payload and checksum may be corrupted in many if notalmost all bits. FIGS. 1B and 1C show the central problem with mostprior art error correction-detection schemes, when an early bit isdeleted from such a message as in FIG. 1B or inserted as in FIG. 1C,these schemes see all the bits that follow as wrong, making errorcorrection impractical.

While there have been sporadic research results for error correction inthe presence of bit insertions and deletions, systematic errorcorrecting codes of more than 8 bits have proved computationallydifficult, according to a recent survey article by N. J. A. Sloane, “OnSingle-Deletion-Correcting Codes”, last updated 2002. As mentioned inthat article, error correction for a channel inserting single bits isvery similar to what was surveyed for deleting single bits. A messageprotocol is needed that can minimize the damage from bit insertion orbit deletion noise for messages much longer than today's 8 bitlimitation discussed in Sloane's article.

SUMMARY OF THE INVENTION

Embodiments of the invention include a message format for a channelexperiencing single bit slip noise. The single bit slip noise may becaused by single bit insertion in some embodiments. Alternatively, thesingle bit slip noise may be caused by single bit deletion. The methodof error correction is not based upon a fundamental breakthrough inerror control coding, but on solving smaller problems that lead to asolution of the large problem.

The message format may include a data payload of N bits, and a correctorcomponent including a first checksum of J=IntLog2 of N bits and achecksum corrector that supports correcting the single bit slip noise.The first checksum may be the sum of each data payload bit multiplied byits position modulo N+1. This allows the use of existing errorcorrecting codes to be applied to the checksum of up to J=8 bits, fordata payloads of up to 127 bits. The received corrector component allowsthe checksum to be correctly derived in the presence of single bitslipping noise. With the corrected checksum, the received data payloadcan be corrected for single bit slipping noise.

In certain embodiments of the invention, the data payload may be largerand the message format may include the first checksum and the correctorcomponent of a second checksum as the sum of each checksum bitmultiplied by its position modulo J+1 to create the second checksum as asingle bit deletion error correcting checksum of the first checksum. Thechecksum corrector is now applied to the second checksum. Decodingproceeds by first recovering the second checksum from the correctorcomponent, then recovering the data payload from the received datapayload and the corrected first checksum.

A second message format supports much larger data payloads of length Nusing three checksums, with the corrector component including the thirdchecksum and a third checksum corrector. Example: Given N<4096, thenJ<=12 and K<=4. Decoding proceeds by first recovering the secondchecksum from the corrector component, then the first checksum from thereceived first checksum and the corrected second checksum, followed byrecovering the data payload from the received data payload and thecorrected first checksum.

Apparatus embodiments of the invention may include a transmittergenerating a transmitted message of this format and/or a receiver usinga received message that may be corrupted from the transmitted messagethrough bit deletion or insertion. The transmitter implements a multiplechecksum process to create the transmitted message from data input. Thereceiver implements a multiple checksum error correction-detectionprocess to generate a received data output from the received message.Both processes are embodiments of the invention.

The transmitter and/or the receiver may include a processor to implementthe processes. Each processor may include at least one instance of afinite state machine and/or a computer. The process embodiments may beimplemented as a program system residing in a computer readable memoryconfigured to be accessed by a computer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a message of the prior art including a data payload of Nbits and a checksum of J bits.

FIGS. 1B and 1C show the central problem with most prior art errorcorrection-detection schemes, when an early bit is deleted from such amessage as in FIG. 1B or inserted as in FIG. 1C, these schemes see allthe bits that follow as wrong, making error correction impractical.

FIG. 2A shows an example of an embodiment of the invention as a messageincluding the data payload and a corrector component of a checksumderived from the data payload with the corrector component generatedfrom the checksum. The received corrector component allows the checksumto be correctly derived in the presence of single bit slipping noise.With the corrected checksum, the received data payload can be correctedfor single bit slipping noise.

FIG. 2B shows another example of an embodiment of the invention as amessage including the data payload, the first checksum as in FIG. 2A anda corrector component of a second checksum derived from the firstchecksum with the corrector component generated from the secondchecksum. The received corrector component allows the second checksum tobe correctly derived in the presence of single bit slipping noise. Withthe corrected second checksum applied to the first checksum, the firstchecksum can be correctly derived in the presence of single bit slippingnoise. And with the corrected first checksum and the received datapayload, the data payload can be correctly derived in the presence ofsingle bit slipping noise.

FIG. 3 shows some example apparatus embodiments of the invention thatmay include a transmitter generating a transmitted message of the formatshown in FIG. 2A and/or a receiver using a received message in thisformat that may be corrupted from the transmitted message through singlebit deletion noise as in FIG. 1B or single bit insertion noise as inFIG. 1C. The transmitter uses a first processor to implement an encodingprocess to create the transmitted message from data input. The receiveruses a second processor to implement a decoding process to generate areceived data output from the received message. Both processes areembodiments of the invention.

FIG. 4 shows that a processor as used herein may include at least oneinstance of a finite state machine and/or a computer accessibly coupledvia a buss to a computer readable memory containing a program system andthe transmitted message and/or the received message.

FIG. 5 shows a flowchart of the program system that may includeimplementations of an encoding process to create the transmitted messagefrom data input and/or of a decoding process to generate a received dataoutput from the received message in the presence of single bit slippingnoise.

FIGS. 6 and 7 show flowcharts of some details of creating thetransmitted message from the data input.

FIGS. 8 to 9 show flowcharts of some of the details of reversecalculating the received data output 18 from the received message 32with error correction-detection based upon the received data, possiblyreceived correction component.

FIG. 10 shows a block diagram example of communications between a firstdevice and a second device, with a communicative coupling potentiallysupporting a bidirectional communications, with the transmitter of theeach device communicating to the receiver of the other device. Note thatin some embodiments of the invention, one of the devices may include asingle processor to support both the transmitter and the receiver ofthat device.

And FIG. 11 shows an example of some refinements of the message formatof FIG. 2, possibly including a preamble and/or possibly using threechecksums, with the corrector component encoding a small enough thirdchecksum to error correct single bit deletions or alternatively, singlebit insertions upon reception across a single bit slipping channel.

DETAILED DESCRIPTION

This invention relates to the correction of data payloads where the mostcommon transmission error is a single bit inserted or deleted, alsoknown as bit-slipping, which is particularly useful in communicationsystems that rely on the synchronization of transmitter and receiverclocks to correctly transfer data payloads.

Embodiments of the invention include a message format including a datapayload of N bits, a first checksum of j=IntLog2 of N bits and a secondchecksum of IntLog2 of IntLog2 of N bits, where the first checksum isthe sum of each data payload bit by its position modulo N+1 and thesecond checksum is the sum of each checksum bit by its position moduloj+1.

Referring to the drawings more particularly by reference numbers, FIG.1A shows a message 2 of the prior art including a data payload 4including an N bits (x₁ x₂ . . . x_(N)) and a checksum 6 of j bits (a₁ .. . a_(j)) which will be referred to as a, which may not be a CRCcoding.

FIG. 2A shows an example of an embodiment of the invention as a message2 including the data payload 4 and a corrector component 10 that encodesthe checksum 6. The checksum may be derived from the data payload andthe corrector component is generated from the checksum. The receivedcorrector component allows the checksum to be correctly derived in thepresence of single bit slipping noise. With the corrected checksum, thereceived data payload can be corrected for that single bit slippingnoise. In certain embodiments of the invention, the corrector componentmay have a fixed checksum associated with it, such as the value 0.

FIG. 2B shows another example of an embodiment of the invention as amessage 2 including the data payload 4, the first checksum 6 as in FIG.2A and a corrector component 10 encoding a second checksum 8. The secondchecksum may be derived from the first checksum and the correctorcomponent may be generated from the second checksum. The receivedcorrector component allows the second checksum to be correctly derivedin the presence of single bit slipping noise. With the corrected secondchecksum applied to the first checksum, the first checksum can becorrectly derived in the presence of that single bit slipping noise. Andwith the corrected first checksum applied to the received data payload,the data payload can be correctly derived in the presence of that noise.

FIG. 3 shows some example apparatus embodiments of the invention thatmay include a transmitter 22 generating a transmitted message 2 and/or areceiver 26 using a received message 32 that may be corrupted from thetransmitted message through bit deletion as shown in FIG. 1B or bitinsertion as in FIG. 1C. The transmitted message and the receivedmessage are in the same formats as shown through the example of FIG. 2A.The transmitter may use a first processor 20 to implement an encodingprocess to create the transmitted message from data input 16. The datainput 16 is used to generate the checksum 6 and to create the datapayload 4. The checksum is used to encode the corrector component 10.The receiver may use a second processor to implement an errorcorrection-detection process to generate a received data output 18 fromthe received message. Both processes are embodiments of the invention.The second processor uses the received corrector component 38 to createa corrected checksum. The second processor then uses the correctedchecksum with the received data payload 34 to create the corrected data40.

FIG. 4 shows that a processor 20 as used herein may include at least oneinstance of a finite state machine 52 and/or a computer 54 accessiblycoupled 56 via a buss to a computer readable memory 58 containing aprogram system 100 and the transmitted message 2 and/or the receivedmessage 32. The program system may reside in a volatile and/or anon-volatile memory component. As used herein, a volatile memorycomponent tends to lose its memory state if it is not supplied power,whereas a non-volatile memory component does not tend to lose its memorystate in the absence power.

As used herein a finite state machine 52 receives at least one inputmaintains at least one state and generates at least one output basedupon the value of at least one of the inputs and/or of at least one ofthe states. The state may be altered by the input, the output or thefeedback of the state within the finite state machine.

As used herein a computer 54 includes at least one instruction processorand at least one data processor with each data processor beinginstructed by at least one of the instruction processor, with at leastone of the instruction processors instructed by the program steps of theprogram system 100.

Some of the following figures show flowcharts of at least one embodimentof the method, which may include arrows signifying a flow of control,and sometimes data, supporting various implementations of theinvention's operations. These include a program operation, or programthread, executing upon a computer 54, and/or a state transition in afinite state machine 52. The operation of starting a flowchart refersentering a subroutine or a macro instruction sequence in the computer,and/or directing a state transition in the finite state machine,possibly while pushing a return state. The operation of termination in aflowchart refers completion of those operations, which may result in asubroutine return in the computer, and/or popping of a previously storedstate in the finite state machine. The operation of terminating aflowchart is denoted by an oval with the word “Exit” in it.

FIG. 5 shows a flowchart of the program system 100 may includeimplementations of a process to create the transmitted message from datainput and/or of a multiple checksum error correction-detection processto generate a received data output from the received message subject tothe single bit slip noise. In particular, program step 102 supportscalculating the transmitted message 2 from the data input 16 of FIG. 3using at least one checksum 6 and the corrector component 10 of FIGS.2A, 2B and/or 12. The corrector component may correct for the single bitslip noise. And program step 104 supports calculating the receivedoutput data 18 from the received message 32 subject to the single bitslip noise with error correction-detection based upon received datapayload 34 and the received corrector component 38.

FIG. 6 shows a flowchart of some details of the program step 102creating the transmitted message from the data input. Program step 110supports creating the data payload 4 of the length N from the data input16 of FIG. 3. Program step 112 supports calculating the first checksum 6from the data payload of length N. Program step 114 encodes the firstchecksum to correct for single bit slip noise to create the correctorcomponent 10.

FIG. 7 shows a flowchart of some details of the program steps 112calculating a checksum for bit data of length M. Program step 120 sets asum to zero given the bit data. Program step 122 iteratively accumulatesover the position p starting from 1 to M of the sum of p multiplied bythe data[p] representing the p^(th) bit in the bit data. Program step124 returns that sum modulo M+1. Note that in some embodiments of theapparatus a computer 24 may be using these program steps as a subroutineor function call, whereas in other embodiments, these operations may beimplemented as a sequence of instructions without any call and returnmechanism involved.

The basic encoding process shown in FIGS. 6 and 7 calculates thechecksum 6 from the data payload 4 represented as a stream of length Nof the bits x_(i) as

$\begin{matrix}{a = {\sum\limits_{i = 1}^{N}{x_{i}{\left( {{{mod}N}\left( {+ 1} \right)} \right.}}}} & (1)\end{matrix}$

Where j=Log₂ (N), the number of bits in a, is the rounded up version ofthe logarithm base 2 of the number of bits N in the data payload 4. Byway of example, if N=256 then j=8.

The encoding 114 creates the corrector component 10 from the firstchecksum 6, which may perform a table look-up, using the first checksumas an index into the table to retrieve the checksum corrector as thetable entry.

FIG. 8 shows a flowchart of some details of program step 104 calculatingthe received output data 18 from the received message 32 subject to thesingle bit slip noise with error correction-detection based upon thereceived corrector component 38. Program step 116 decodes a correctedchecksum 19 from the received corrector component to correct for thesingle bit slip noise that may be in the first second checksum. Programstep 118 reverse calculates the received data output 18 from thereceived data payload 24 and from the corrected checksum.

FIG. 9 shows a flowchart of the program step 114 of FIG. 8 that reversecalculates from a corrected data 40 of length M. Program step 130calculates the checksum a′ of the received data of length M. Denote thereceived data as (y₁ y₂ . . . y_(M)) and that the received checksum isa. Calculate the checksum of the received data payload as

$\begin{matrix}{a^{\prime} = {\sum\limits_{i = 1}^{M}{y_{i}{\left( {{{mod}\; M} + 1} \right)}}}} & (2)\end{matrix}$

Continuing with the flowchart of FIG. 9, program step 132 determines ifthe received checksum a equals the checksum a′ of the received data,then no errors are detected in the transmission and the corrected datais set to the received data by program step 134. Program step 136supports the otherwise situation by using the received checksum, thereceived data, and the checksum of the received data to create thecorrected data 40.

Program step 136 may be to determine which bit has either been insertedor deleted in the received data payload 34. Determining which bit hasbeen deleted is directly found in Sloane's paper and determining whichbit has been added is similar to the discussion in that paper.

FIG. 10 shows a block diagram example of communications between a firstdevice 50 and a second device 50, with a communicative coupling 24potentially supporting a bidirectional communications, with thetransmitter 22 of the each device communicating to the receiver 26 ofthe other device.

The devices 50 may include transmitters 22 and receivers 26 separated bylarge distances, such as a satellite as a first device 50 communicating24 with a ground station as the second device 50, which may be a fixedposition facility, a moving vehicle or a mobile communications devicesuch as a portable communications terminal or telephone. Thecommunicative coupling 24 may include at least one wireless physicaltransport. The device 50 may include a single processor 20 to create thetransmitted message 2 and process the received message 32 into thereceived data output 22.

Note that in some embodiments of the invention, the length of the datapayload 4 may be included as part of a message. In some situations thatlength may be part of the message for that payload. In other situationsthe length of the data payload may be sent in a preceding message.

FIG. 11 shows some variations in the format of the message 2, which mayfurther include a preamble 3 that may act as a synchronization sequence,often composed of a long string of a constant bit such as ‘0’. This maybe sent by transmitters to be used by the receivers to detect the startof the message and is usually not considered part of the message forwhich error correction is applied.

A refinement of the message 2 as shown in FIGS. 2A and 2B supports muchlarger data payloads 4 using more than two, for example three checksums,the first checksum 6 as before, the second checksum 8 as the sum of eachfirst checksum bit multiplied by its position modulo J+1. The length ofthe second checksum will be referred to as K. The third checksum 9 isderived from the second checksum bits and with the corrector component10 corrects the single bit slip noise to create a corrected thirdchecksum. Example: Given N<4096, then J<=12 and K<=4.

The preceding embodiments provide examples of the invention, and are notmeant to constrain the scope of the following claims.

1. A communication system, comprising: a transmitter sending atransmitted message via a communicative coupling to a receiver to createa received message that may be corrupted by single bit slip noise, withsaid transmitted message including a data payload of length N and acorrector component with a checksum of length J and a correctorcomponent encoding said checksum to correct said single bit slip noisein said checksum to correctly determine said checksum, with saidchecksum as the sum of the data payload bits multiplied by their payloadpositions modulo N+1.
 2. The communication system of claim 1, furthercomprising a first device including said transmitter, and a seconddevice including said receiver.
 3. The communication system of claim 2,wherein said first device is a satellite and said second device is aground station.
 4. The communication system of claim 1, wherein saidsingle bit slip noise effects a single bit deletion in said transmittedmessage.
 5. The communication system of claim 1, wherein said single bitslip noise effects a single bit insertion in said transmitted message.6. A device, comprising: a receiver including a processor configured tooperate upon a received message subject to single bit slip noiseaffecting a transmitted message including a data payload of length N anda corrector component including a checksum of length J and a correctorcomponent encoding said checksum to correct said single bit slip noisein said checksum to correctly determine said checksum, with saidchecksum as the sum of the data payload bits multiplied by their payloadpositions modulo N+1.
 7. The device of claim 6, wherein said receivedmessage includes a received data payload, a received checksum and areceived second checksum; wherein said processor may include at leastone instance of at least one member of the group consisting of a finitestate machine and a computer accessibly coupled with a computer readablememory including a program system.
 8. The device of claim 7, whereinsaid processor is configured to decode a corrected checksum of saidlength J based upon said received second checksum and said correctedchecksum and reverse calculate said received data payload of said lengthN based upon said corrected checksum ands said received data payload tocreate a received data output corrected of said single bit slip noise.9. The device of claim 6, wherein said single bit slip noise effects asingle bit deletion in said transmitted message.
 10. The device of claim6, wherein said single bit slip noise effects a single bit insertion insaid transmitted message.
 11. A device, comprising: a processorconfigured to create a transmitted message that may be corrupted bysingle bit slip noise, with said transmitted message including a datapayload of length N and a corrector component with a checksum of lengthJ and a corrector component encoding said checksum to correct saidsingle bit slip noise in said checksum to correctly determine saidchecksum, and with said checksum is the sum of the data payload bitsmultiplied by their payload positions modulo N+1.
 12. The device ofclaim 11, wherein said processor may include at least one instance of atleast one member of the group consisting of a finite state machine and acomputer accessibly coupled with a computer readable memory including aprogram system.
 13. The device of claim 12, wherein said processor isconfigured to calculate said checksum as the sum of the data payloadbits multiplied by their payload positions modulo N+1, and encode saidcorrector component to correct sad single bit slip noise in saidchecksum to correctly determine said checksum.
 14. The device of claim11, wherein said single bit slip noise effects a single bit deletion insaid transmitted message.
 15. The device of claim 11, wherein saidsingle bit slip noise effects a single bit insertion in said transmittedmessage.
 16. A method, comprising at least one of the steps of:generating a transmitted message including a data payload of length Nand a corrector component encoding a checksum of length J o correct theeffect of single bit slip noise on said checksum; and generating areceived data output from a received message from said transmittedmessage possibly corrupted by said single bit slip noise, furthercomprising the steps of decoding a corrected checksum with a receivedversion of said corrector component to create a corrected checksumwithout said single bit slip noise; and reverse calculating using areceived version of said data payload and said corrected checksum tocreate said received data output as said data payload without saidsingle bit slip noise.
 17. The method of claim 16, wherein the step ofgenerating said transmitted message including said data payload oflength N and said corrector component, further comprises the steps ofcalculating said checksum from said data payload of said length N; andencoding said corrector component from said checksum to correct for saidsingle bit slip noise.
 18. The method of claim 16, wherein said singlebit slip noise effects a single bit deletion in said transmittedmessage.
 19. The method of claim 16, wherein said single bit slip noiseeffects a single bit insertion in said transmitted message.